Make sure righty doesn't find out about lefty or he may get jealous
OT: If your girlfriend is okay with it, sure. Open relationships are cool... I guess. But if you just go around cheating on a bunch of people, you're a douchebag.

From a woman's perspective, if I found out that the person I was dating was also dating other people, secretly or not... well, I'm generally a calm person but I can be quite vicious when I feel the need to be. There is no stability in a romantic relationship involving more than 2 people.

In Soviet Russia Hell still hath no fury like a woman scorned...

Seriously though - regardless of the posters sincerity - as long as all parties are in on it, its okay.

I have nothing against polygamy, never understood why the system has something against it. Go ahead if you both are okay with it. But how do you see this relation? As in:
- One guy, multiple girls
or
- Multiple guys, multiple girls (which seems more fair to me, to be honest)
Problem is, you might start to see your other men and women in relation as a kind of family, in a certain way; instead of just a multitude of wives and husbands. That is a feeling that you might search to avoid.
But again, if you have no problem with multiple wives (and if your wives have multiple husbands), go ahead! It just means some people are happier then on this earth.

Anyways, multiple GF's just sounds too expensive. I don't really see the gain.

if cohabiting the gains on sex wouldnt necessarily be substantial. women cohabiting have a tendency to have their periods get in sync (hormones and all), so no gains there. and let's face it, there is only so much sustained sex the male body is capable of, in spite of macho claims otherwise. and the picture of multiple women on periods, or the prospect of any one of them getting into snits and having to placate is not a good one. not that i dont get in snits, but there is only one of me to put up with in that situation, lol